1. Technical Field
The invention relates to a method and apparatus for forming end shells for metal containers, and particularly to forming end shells having an annular reinforcing rib. Even more particularly, the invention relates to forming the reinforcing rib in the end shell in a single stroke and at a single station of a double action press.
2. Background Information
In the metal container art, containers usually consist of a body formed of lightweight metal, such as aluminum, and a separate end shell for closing the container, also formed of lightweight metal, from strip material. It is desirable in forming the end shell to form a reinforcing rib at the junction of the chuckwall with the central panel of the shell to provide strength and rigidity to the end shell when secured to the container body. Heretofore, these reinforcing ribs were formed as annular grooves in the end panel, examples of which are shown in U.S. Pat. Nos. 4,713,958, 4,715,208, 4,716,755, 4,808,052, 4,587,825, and 4,516,420. Although these types of reinforcing ribs have proven satisfactory, they provide an area in the can end which can collect impurities and other materials. This is especially undesirable when the container has a removable tab which enables the contents to be drank directly from the container.
Another type of reinforcing rib, referred to as a rolled rib or folded rib, has been developed to replace the annular groove reinforcing rib. This reinforcing rib is formed in the metal end shell by collapsing or rolling a portion of an unclamped portion of an end shell chuckwall back upon itself during the formation of the end shell in a single action press. Although this type of rolled or folded reinforcing rib has proven satisfactory, it involves multiple operations and in particular, requires forming the folded rib at a first station or in a first press, and then final forming it in a second station or second press due to the partial unrolling or unfolding of the rolled rib after it has been formed at a first station due to the tendency of the metal in an unclamped portion thereof to return to its pre-stamped position.
Thus, the need exists for an apparatus and method for forming container end shells having a folded or rolled annular reinforcing rib in a single press cycle and at a single station, without having to complete the forming of the rib at a subsequent station, by eliminating the tendency of the stamped end shell including the rolled rib, to return to its pre-stamped condition.